(1) Field of Invention
The present invention relates to the formulation and manufacture of an emulsion adhesive with a solids content greater than 60% solids that is easily coatable or spreadable on substrates and exhibits excellent stability upon aging. The present invention teaches a method to increase the solids content of an existing emulsion by forming solid particles within the water phase of an existing emulsion so as to create multimodal solids distribution of particles thereby significantly increasing the packing density and thereby the solids content of the resulting emulsion with a special inversion process. The present invention in particular relates to a method of making and using such stable water based emulsion adhesives to avoid paper substrate wrinkling. The present invention also in particular relates to the avoidance or a drastic reduction of the water drying step in the manufacture of coatings from such inventive high solids emulsion adhesives.
(2) Description of Related Art
The upper limit of solids content of water based emulsion adhesives is generally about 60% as it very difficult to make any higher solids by present methods. At this solids level, emulsion adhesives still have a significant amount of water in them. As a result one has to be careful in directly coating such adhesives onto paper where the water may be absorbed and cause the paper substrate to curl and deform.
The presence of large amount of water in typical emulsions does not allow direct coating of emulsions onto a paper substrate without wrinkling the paper. As a result, transfer coating methodology is usually used to coat a water-based pressure sensitive adhesive onto a paper substrate. The adhesives are first coated directly onto a transfer film or silicone coated paper where the absorption of water is kept to a minimum. The adhesive is then dried. The adhesive coating is then transferred to a paper substrate where the paper now sees a much lesser water content and is thus not subjected to any water absorption and subsequent curling. The intermediate transfer substrate is typically a silicone coated paper or film and is usually a wasted by-product. There is great utility in reducing the amount of water in emulsions, by making them higher solids, to reduce wrinkling of paper substrates when such adhesives are directly coated onto paper, thereby by-passing a transfer coating step.
Large amounts of energy are also typically needed to dry water based emulsion adhesives. Removal of water is an energy intensive process. It is useful to minimize the energy used in this drying step by reducing the water content and yet get good adhesion characteristics. Both the consumption of energy for drying and the spewing of dryer exhaust into the atmosphere are fundamentally environmentally unfriendly. As a result, there is great utility in creating water-based adhesives with solids content as high as possible.
There is an upper limit to the level of solids of a stable emulsion adhesive. The theoretical maximum packing ratio of solids in emulsion or suspensions in a liquid is around 63%. In general it is difficult for industrial emulsion adhesives to achieve such a high packing ratio. In practice solids content above 55% are usually obtained with a great deal of careful recipe engineering in the case of emulsions. In general, there has to be a very careful balance of surfactant level and the surfactant type in recipe design.
U.S. Pat. No. 4,687,594 teaches the design of such high solids polyvinyl acetate emulsions. While vinyl acetate emulsions are not pressure sensitive adhesives by themselves, they may be components of pressure sensitive emulsion adhesives, in combination with other materials like acrylics. Here they are discussed to show limits of level of solids in emulsions. Vinyl acetate copolymer emulsions having a solids content of 65% by weight are known and some are articles of commerce. Polyvinyl acetate emulsion has rigid non-tacky particles as inner phase and the coagulation tendency is not as severe as in a pressure sensitive adhesive emulsion which has a tacky inner phase. These emulsions, however, often require high levels of surfactant or protective colloid to achieve the required stability to shearing forces and to freezing. In addition, special techniques as delayed surfactant addition or the use of monomer pre-emulsions are often required in order to achieve manageable viscosities in production. These departures from conventional practice, while successfully producing 65% solids latexes, can lead to undesirable properties such as reduced resistance to moisture and unsatisfactory rheological properties. The variations from conventional procedures may also require equipment changes and result in a more expensive process. It is difficult to manage solids levels in emulsions beyond 65% solids and obtaining a cost effective stable and coatable emulsion adhesive. As a result, one very seldom sees in the market water based emulsions at greater than 65% solids.
U.S. Pat. No. 4,767,813 also teaches a way of forming high solids aqueous based pressure sensitive adhesives by directly emulsifying the tackifier into a water based emulsion by mechanically mixing an additional oil phase into the existing water phase of the base emulsion. This process and subsequent increase in level of solids is limited by the agitation means and the ability to breakdown the tackifier phase into smaller particles for incorporation into the continuous water phase of the emulsion. The mechanical breakdown and incorporation of tackifier into the water phase of the base emulsion will increase solids % but once again be limited in packing density and hence the overall solids content by the ability to break down the tackifier mechanically into small domains. A more innovative way to incorporate a tackifier into the emulsion mix is desirable.
The term ‘ultra-high’ solids denotes % solids level in water-based emulsions in excess of 60% solids. In fact, for the purposes of this invention the meaning of the term ‘ultra-high solids’ is a solids range from 60% to about 90%. In the art, U.S. Pat. No. 6,001,916 defines the same term to mean solids in excess of 65%, preferably greater than 70%.
High solids pressure sensitive adhesives present huge opportunities in the commercial world by reducing and eliminating, if solid are high enough, a production step of drying and the corresponding investment in drying equipment. Hence with such an adhesive one can easily apply an adhesive to select areas of a substrate in a printing machine without the need for ovens to subsequently dry the adhesive. Currently hot-melts can be printed in such a fashion, but no water-based adhesive are available to do this. Hot melt adhesive requires controlled heating equipment to melt the adhesive for coating and therefore is cumbersome to use. This adhesive is room temperature coatable and requires minimum heating to dry into a tacky film.
Emulsions obtained by the processes of prior art still have significant water content that needs a subsequent drying step to form good adhesive films. What is desired is a process to further increase the solids, while maintaining formulation stability, to the extent that the water content of the resultant adhesive is so low, preferably 70% solids or above, that one does not need a separate drying step to form adhesive films and at the same time there is no significant paper curl from moisture penetrating into a paper coated substrate.
In general there is always a utility for a water based emulsion adhesive with lower water content, i.e. a very high solids content, preferably above 60%, to reduce the energy consumption in drying of coated adhesive films. There is also a great need to have high solids emulsions that can be applied to most papers without causing significant wrinkling or curling of the paper and can be dried at room conditions with minimal heat and/or additional airflow.